Mesicopter Progress Report
October 1999

Summary

See the September report for the previous update.

Rotor Development

Aerodynamics (Kunz, Kroo)

A new rotor design based on the 5mm Smoovy was developed and passed on to Shelly. The rotor should be able to produce up to 4g of thrust (16g for 4), making the electronics much more reasonable in the near term. Because of the high RPM at which the motor likes to operate, the optimal diameter for direct drive remains quite small, this means the rotor solidity is much higher than for our previous designs. The new 5-bladed rotor is shown in Shelly's CAD image below. We are also preparing to design new rotor for the WesTek motors based on Peter's new data.

The calculation of airfoil section lift and drag from the 2-D CFD results has been modified to utilize a control-volume approach. This integrates the pressure forces and momentum flux around a closed curve encircling the airfoil. The calculation of lift and drag had been based on surface pressure integration. This generally works well for conventional airfoil geometries, but for the as-built geometries, with blunt leading and trailing edges, this method has proven to be very inaccurate due to the strong influence of the integrated edge forces on the drag of the section. Surface pressure integration results become inconsistent and highly grid dependent. The control-volume calculation provides much more consistent values without resorting to extremely fine grid resolution. Initial results with this new method in place indicate that while there is still a significant drag penalty for these as-built sections, it is not as severe as previously indicated by the pressure integration results. Work is continuing on developing a geometry that is manufacturable while minimizing the cost in aerodynamic performance.

Fabrication (Cheng)

A new rotor design CAD model for the 5mm Smoovy motor was developed (see figure below). The rotor is 22mm in diameter with a 2mm-diameter hub in the center. The cross-section of each blade is a NACA4402, and modified to NACA4404 at positions less than 18% of radius for increased strength. By assigning chord length and twist angle at 21 positions from the center to the tip, the shape of blade is created by interpolation.


SEM and optical pictures of our previous blades, shown in the figure below, indicate that the section shape did not match the intended airfoil sections. These sections for the 3mm motors were about 80 mm thick and it is uncertain that regions less than 50-80 mm thick can survive during the CNC machining.

Experiments are being conducted to understand the limitations of machining with the current materials. The first trial was based on the optimal design. The second trial changed the building direction and machining strategy in order to obtain the exact shapes.

	Trial 1	Trial 2
Building Direction	+Z axis	-Z axis
Rotor surface	Convex	Concave
End mill used	Ball	Ball and flat
Strategies	Machine bottom surfaces à Cast polymer à Machine top surfces	Use both ball and flat end mills to create exact concave shapes
Problems	Need over-cut to obtain exact shape. The gap between blades are too small for over-cut. Flush at the edges.	Yellow and purple wax substrates are easily chipped off when the walls (gaps between blades) are very thin. Connections between blades and the hub are not strong enough. Flush at the edges.
Need to improve	Shape Flash	Connections between the hub and blades Flash Warpage of epoxy rotor

Because of the existence of flash, it is hard to determine if thin regions can survive during machining. Therefore, later experiments will focus on the flash removal and thickness limitation. Additional validation of the as-built shapes will be made as new blades and processes are developed.

Motor System

Motors -- Experimental Testing (Kunz)

Experimental motor performance testing has been completed on two motors being considered for different aspects of this project. The WES-Technik DC5-2.4 is a 10g motor with an advertised maximum power output of 2.4 Watts and a maximum efficiency of 75%. Results of testing at the 5V nominal voltage confirm the advertised maximum power output. The measured efficiency was also very close to the manufacturer's values. This is considerably more efficient than most motors in this size and output power range. For initial prototyping of the larger vehicle, several other motors are also currently being considered. Although these motors are less efficient than the DC5-2.4, they generally operate at between 1.5 and 3 volts as opposed to 5 volts for the DC5-2.4. This would facilitate the use of a simpler and potentially lighter power supply.

Test data has also been compiled on the Smoovy 5mm motor, but further experimentation is required before an accurate assessment of the motor can be completed. The bearing friction of the test rig introduces a zero offset into the data. For larger motors this represents a very small percentage of the output torque, but for the small torque values of the 5mm Smoovy, this offset can be as large as 20% of measured torque. The relative changes in torque versus RPM are valid for any given data run, but the initial friction offset is currently not known. A single accurate point would provide the necessary data to calibrate the completed runs

Motor Control Electronics (Fabian)

The functionality of the Philips motor controller TDI5145 has been tested. Simplest motor controller circuit consisting of TDI5145 and four capacitors has been built and successfully used to spin a 3mm Smoovy motor. The standard package of the TDI5145 weights several grams, therefore a surface mount package of TDI5145 (exact part number TDI5145T) was searched for. This chip is currently not available and can be ordered with minimum order of several thousand pieces. A replacement in form of TDI5144T was chosen. It is a TDI5145T with reduced functionality but still useful for our purpose. A first PCB design for the mesicopter has been done.
The possibility of a voltage doubler for the power supply from battery cells has been considered. Samples of two different DC-DC controller types and the external circuitry were ordered and compared. The first type uses a coil as an element for energy conversion/storage. This type is capable of large output voltage range and also large output currents. The drawback is the size and weight of the required coil. The second DC-DC controller needs only capacitors for its operation. The drawbacks are discrete number of possible output voltages and small output currents. An own design of the second type controller is also considered.

Battery System (Brunet)

Due to the minimum voltage required to drive the motors and control electronics, and the limited commercial battery options for testing, the use of dc to dc voltage converter was considered. By using a dc-dc voltage converter we can obtain higher voltages with less weight than would be possible using commercial batteries in series. These converters, or pumps, consist of capacitors (or inductors) which are discharged at a rate controlled by a switcher. The pumps are able to provide constant voltage at an efficiency of up to 85%. They also provide constant current which is a benefit for driving the motors. Our search for commercial batteries for use in the on-going development efforts continues. Contacts have been made with several manufacturers and we have tested Varta Nickel Metal-Hydride rechargeable button cells, Rayovac Silver Oxide ultra high drain cells, and Sanyo Nickel Cadmium cells. In initial testing on a circuit which simulates the resistance of the motor system the NiMH cells showed promising results. Further testing is required.

Structures/Frame Development (Prinz, Cheng)

Our focus has been on other aspects of the system this month, but some discussions about layout and PC-board development have been on-going. Some frame design ideas have served as examples for our new people to become familiar with the Unigraphics CAD system.

Stability and Control (Fay)

The 2-D linearization of the mesicopter has been completed. A comparison between the response of the previous nonlinear model and that of the linearized model indicates horizontal position and pitch of the vehicle are well modeled by the linearization. Vertical position, however, seems to be a higher order phenomenon and is not captured in the linearization. The rotor speeds for these simulations were fixed, so all dynamics arise from introducing disturbances away from the equilibrium position.

Using the linearized model, system poles were calculated. An initial study on the affect of various parameters on these poles has begun. Currently the only variables in the model are the location of the center of gravity and the tilt angle of the rotor shafts. Plotting the poles against these two parameters indicates that there may be an optimal location for placing them with regards to stability, robustness, and speed of response.

Sensors and Communications (Kroo, Holden)

Mike showed his prototype MAV flight controller that incorporates a rate gyro, pressure sensor (for altitude), and GPS along with a programmable microcontroller in a very tiny package. While the 3g and 15g mesicopters will not be able to incorporate this kind of sensing for a while, the larger WesTek device probably will. More details next time.

System and Application Issues (Kroo)

Met with Sam Wilson (DARPA MAV Program Manager) and NASA Ames researchers (Larry Young, Ed Aiken) to discuss nearer-term applications and some of the communications and control issues. Continued discussions with JPL (Bob Balaram) on Mars opportunies and Langley (Dan Moerder) on stability and control issues. Larry Young has gotten the American Helicopter Society to focus its annual design contest on a small autonomous rotorcraft for Mars exploration.

In the meantime, it seems we are not the only ones looking at very tiny aerial robots. A recent article in the Guardian mentions a rather hard to believe Chinese version of the mesicopter, while the San Francisco Chronicle carried a story about our friends in Berkeley and their (let's just say) ambitious (and well-funded) efforts to build a tiny ornithopter. For something closer to reality, check out this picture of what some claim is the world's smallest radio-controlled airplane.